One would have to be extremely cynical to argue that Nationwide is not a force for good in the financial services sector.

Certainly, with 15 million members, it must be doing something right. One reason it attracts such loyalty is the way, a decade ago, it ripped up the old model beloved of traditional mortgage lenders, under which new customers were subsidised by the old book of business.

The strategy — advertised on TV by the actor Mark Benson parodying the way banks give their best offers to “new customers only” — led